import java.util.Scanner;

public class MortgageCalculator
{

	private final static int ASSUMED_YEARS = 20;
	private final static int MONTHS_IN_YEAR = 12;
	private final static int APR_CONVERSION_CONSTANT = 100;

	private final static String WELCOME_PROMPT = "Welcome to the mortgage calculator.";
	private final static String PRINCIPAL_PROMPT = "Please enter the mortgage amount: ";
	private final static String APR_PROMPT = "Please enter the annual interest rate (APR): ";
	private final static String ASSUMING_X_YEARS_PROMPT = "Assuming a "+ASSUMED_YEARS+" year term, the monthly repayments would be ";
	private final static String AFFORD_TO_PAY_PROMPT = "How much can you afford to pay per month? ";
	private final static String TIME_TO_PAY_USING_SPECIFIED_AMOUNT_PROMPT = "If you pay %s per month your mortgage would be paid off in %s";
	private final static String INVALID_DOUBLE_INPUT_PROMPT = "Invalid input! Please input a number greater or equal than 0!";
	private final static String RUN_AGAIN_KEYWORD = "yes";
	private final static String QUIT_KEYWORD = "no";
	private final static String GO_AGAIN_PROMPT = "Would you like to use the mortgage calculator again ("+RUN_AGAIN_KEYWORD+"/"+QUIT_KEYWORD+")? ";
	private final static String INVALID_GO_AGAIN_INPUT_PROMPT = "Invalid input! Please type '"+RUN_AGAIN_KEYWORD+"' or '"+QUIT_KEYWORD+"'!";

	private static Scanner input = new Scanner(System.in);
	private static boolean quit = false;


	public static void main(String[] args)
	{
		System.out.println(WELCOME_PROMPT);
		while(!quit)
		{
			double principal = readDoubleFromUser(PRINCIPAL_PROMPT);
			double apr = readDoubleFromUser(APR_PROMPT);
			System.out.println(ASSUMING_X_YEARS_PROMPT+String.format("%.2f",calculateMonthlyRepayment(principal, ASSUMED_YEARS, apr)));
			double ammountCanAffordToPayPerMonth = readDoubleFromUser(AFFORD_TO_PAY_PROMPT);
			System.out.printf(TIME_TO_PAY_USING_SPECIFIED_AMOUNT_PROMPT+"\n", ammountCanAffordToPayPerMonth, calculateMonthsToRepayMortgage(principal, ammountCanAffordToPayPerMonth, apr ));
			System.out.println(GO_AGAIN_PROMPT);
			goAgainPrompt();
		}
		input.close();
	}


	private static double readDoubleFromUser(String prompt)
	{
		System.out.println(prompt);
		double number = -1;
		if(input.hasNextDouble())
		{
			number = input.nextDouble();
			if(number >= 0)
			{
				return number;
			}
			else
			{
				return readDoubleFromUser(INVALID_DOUBLE_INPUT_PROMPT);
			}
		}
		else
		{
			input.next();
			return readDoubleFromUser(INVALID_DOUBLE_INPUT_PROMPT);
		}
	}


	private static double calculateMonthlyRepayment(double amount, double duration, double APR)
	{
		double rate = APR/MONTHS_IN_YEAR/APR_CONVERSION_CONSTANT;
		return  (amount * rate) / (1 - Math.pow(1 + rate, -duration*MONTHS_IN_YEAR));
	}

	private static String calculateMonthsToRepayMortgage(double principal, double monthlyRepayment, double APR)
	{
		double base = (1+APR/MONTHS_IN_YEAR/APR_CONVERSION_CONSTANT);
		double x = (1-((principal * (APR/MONTHS_IN_YEAR/APR_CONVERSION_CONSTANT)/monthlyRepayment)));
		int monthsInTotal  = (int)Math.ceil(-log(base , x));

		int years = monthsInTotal/MONTHS_IN_YEAR;
		int monthsLeftOver = monthsInTotal-(years*MONTHS_IN_YEAR);

		return years+(years == 1 ? " year ":" years ")+"and "+monthsLeftOver+(monthsLeftOver == 1 ? " month ":" months ");
	}

	private static double log(double base, double x)
	{
	    return (double) (Math.log(x) / Math.log(base));
	}

	private static void goAgainPrompt()
	{

		if(input.hasNext(RUN_AGAIN_KEYWORD))
		{
			input.next();
		}
		else if(input.hasNext(QUIT_KEYWORD))
		{
			quit = true;
			input.next();
		}
		else
		{
			input.next();
			System.out.println(INVALID_GO_AGAIN_INPUT_PROMPT);
			goAgainPrompt();
		}
	}
}